The invention relates generally to actuators and more particularly to flexural arrangements for supporting a movable member for controlled long range motion relative to a stationary member.
In many micromachine applications that use actuators, design goals include providing a long range of translator motion along a particular axis, while retarding out-of-plane displacements and in-plane displacements that are perpendicular to the intended direction of travel. Electrostatic surface actuators for use in micromachine applications are known. Such actuators may be used in advanced data storage applications and optical telecommunications applications. U.S. Pat. No. 5,986,381 to Hoen et al., which is assigned to the assignee of the present invention, describes electrostatic actuators and voltage variation techniques for driving a translator relative to a stator. U.S. Pat. No. 5,378,954 to Higuchi et al. also describes electrostatic actuators.
All of the actuators displace one element (i.e., the translator) relative to another element (i.e., the stator) and require that the moving element be stabilized against motions in undesired directions. Rolling or sliding elements are most commonly used to directionally stabilize the translator. However, in micromachined actuators, bending flexures are preferred, since the surface contact associated with the rolling or sliding elements is particularly unpredictable and risky for extremely small devices. Nevertheless, bending flexures pose different problems, since their stiffness varies with displacement. For a unidirectional actuator, the bending flexures must stabilize the translator with respect to the out-of-plane motions and with respect to the in-plane motions perpendicular to the desired direction of travel. Supports that are rigid against out-of-plane displacements are particularly important for electrostatic surface actuators, because the force tending to attract the two surfaces is approximately the same as the force pushing the translator parallel to the stator.
FIG. 1 shows an example of one type of folded beam flexure 10 that is used in micromachined electrostatic actuator applications. The flexural device includes a rigid floating beam 12 having opposite sides connected to flexible beams 14 and 16. The flexible beams 14 and 16 are anchored to supports 18 and 20 on a stationary member (not shown), such as a semiconductor substrate. In addition to the pair of flexible beams 14 and 16, there is a second pair of flexible beams 22 and 24 connected to the rigid floating beam 12. The second pair of flexible beams supports a second rigid beam 26, which moves with the movable member of interest.
The folded beam flexure 10 of FIG. 1 produces generally straight-line motion along an x axis. However, as the second rigid beam 26 is displaced along the x axis, the flexible beams 14, 16, 22 and 24 become increasingly compliant to forces along the y axis. For many electrostatic actuators, such as comb drives and surface drives, the applied force in the desired direction of motion (i.e., along the x axis) is accompanied by an instability in the y axis direction. The maximum stable travel of the second rigidly floating beam 26 is thus limited to when the y axis force gradient exceeds the y axis stiffness of the flexible beams.
A second known flexural device 28 is shown in FIG. 2. Here, the translator is connected directly to the rigid floating beam 12 that is connected to the stator (not shown) via the two flexible beams 14 and 16. This device is better suited for maintaining stiffness along the y axis as the rigid beam is displaced. Unfortunately, motion of the rigid floating beam does not follow a straight line as it is displaced. This is not suitable for use in actuators that require straight-line motion.
FIG. 3 shows a third known flexural device 30 which uses micromachine flexible beams. A series of pre-bent flexible beams 32, 34, 36, 38, 40, 42, 44 and 46 is used to provide increased stiffness to motion along the y axis. The flexible beams 32, 34, 36 and 38 are anchored to supports 50 at one end and are connected to either a first rigid floating beam 48 or a second rigid floating beam 52 at the opposite ends. The supports 50 are locations on a stationary member, such as a semiconductor wafer. The flexible beams 40 and 42 connect the first rigid floating beam 48 to a third floating member 54, while the flexible beams 44 and 46 connect the second rigid floating beam to the third floating member.
The flexible beams 32, 36, 42 and 44 are pre-bent in such a way that the ends opposite to the rigid floating beams 48 and 52 are offset in a xe2x80x9cnegative directionxe2x80x9d along the x axis with respect to the connections to the rigid floating beams. On the other hand, the flexible beams 34, 38, 40 and 46 are pre-bent in such a way that the ends opposite to the rigid floating beams 48 and 52 are offset in a xe2x80x9cpositive directionxe2x80x9d along the x axis with respect to the connections to the rigid floating beams. Because of the difference in pre-bent conditions, as one set of flexible beams softens with increased bending, the other set is straightening, thereby maintaining the y axis stiffness. More particularly, as the third floating member 54 which is connected to the translator moves in the positive x direction, the flexible beam 44 will begin to straighten, while the flexible beam 46 will become increasingly bent. This has the effect of causing the third floating member 54 to rotate in a clockwise direction. However, for the same motion, the flexible beam 40 will become increasingly bent and the flexible beam 42 will straighten, causing the third floating member to be urged in a counterclockwise rotation. The two rigid floating beams 48 and 52 are linked at their centers by a bending flexure 56 that acts as a torsional joint. Because each side of the third floating beam 54 is acted upon by offsetting rotational forces, the third floating beam 54 moves in a straight-line motion.
There are some concerns with the flexural device 30 of FIG. 3. First, it does not efficiently use semiconductor wafer area, which is often times at a premium. Second, the flexural device 30 requires contacts to rigid supports 50 which are sometimes fully surrounded by the various beams. Thus, it may be difficult to fabricate the device using a single material layer without external supports, as is often preferred with deep reactive ion etching.
A more complete flexural system 58 that utilizes folded beam arrangements 60 and 62 is shown in FIGS. 4 and 5. Each of the folded beam arrangements 60 and 62 is identical to the one described with reference to FIG. 1, but with a translator 64 taking the place of the second rigid beam 26. The system 58 is shown in a relaxed state in FIG. 4. In this state, the flexible beams 14, 16, 22 and 24 are generally perpendicular to the floating rigid beams 12. As noted above, the flexible beams 14 and 16 are anchored to supports 18 and 20, respectively, on the stationary substrate 65, which is represented by dashed lines. The flexible beams 22 and 24 are connected between the rigid floating beams 12 and the translator 64.
Before any motion takes place in the x direction, the system 58 is very stiff to forces in the y direction. This is because the flexing beams 14, 16, 22 and 24 are straight beams and must buckle in order to allow motion in the y direction. However, the system is more susceptible to forces in the y direction after some displacement of the translator 64 has occurred from the condition shown in FIG. 4. A displaced translator 64 is represented in FIG. 5. Electrostatic forces that urge the translator 64 laterally cause the two rigid floating beams 12 to move more closely together, as indicated by the difference between the dotted lines and the solid lines representing the rigid floating beams. Because of the displacement of the rigid floating beams relative to the anchored supports 18 and 20, a lateral force Fy exerts a moment M as indicated in FIG. 5. The two floating rigid beams are moved in the positive x direction, but the lower beam is also moved in the positive y direction, while the upper beam is moved in the negative y direction.
In the displaced condition of FIG. 5, the flexible beams 14, 16, 22 and 24 are curved. The system is now more susceptible to unwanted displacement in the y direction.
Similar to the in-plane stiffness, the out-of-plane stiffness (i.e., stiffness that is perpendicular to both the x and y axes) depends on the beam displacement. Considering a single one of the flexible beams 14, 16, 22 and 24, the beam can be made extremely stiff with respect to out-of-plane (i.e., z axis) displacements by fabricating the beam with a large aspect ratio (xcex1) equal to the thickness-to-width ratio (t/w). The beam allows displacements along the x axis by bending perpendicularly with respect to its narrow width, as measured along the x axis. Unfortunately, as the flexible beam is bent, the out-of-plane stiffness kz is significantly reduced. It has been theorized that the kz can be related to the in-plane stiffness kx as follows:
kx/kz=1/xcex12+A(xcex4x/L)2xe2x80x83xe2x80x83Eq. 1
where A≈0.280 for aspect ratios greater than 10 and xcex4x is the amount of displacement along the x axis. When the beam is not displaced, xcex4x will be 0 and kz will be much larger than kx. As the beam is displaced, the out-of-plane stiffness is reduced as the square of the displacement period. For a 40:1 aspect ratio beam, a lateral displacement of only 5% of the flexure length causes kz to be reduced by approximately 50%. The stiffness reduction occurs until the bending beam can no longer counteract the out-of-plane force of the actuator drive. The available lateral travel is therefore limited by the reduction in kz with lateral displacement.
As can be seen from Equation 1, it is possible to increase the lateral travel by increasing the lengths of the flexible beams. However, for many applications, it is important to locate the actuators as closely together as possible. Increasing the beam lengths requires the neighboring actuators to be spaced further apart. Therefore, it is more desirable to increase the range of lateral travel without increasing the beam lengths.
The above-identified patent to Hoen et al. describes a technique for offsetting the attractive force that is generated as a result of electrostatically driving the translator. FIG. 6 is a bottom view of a translator 66 which is supported at its four corners by folded beam flexures 68, 70, 72 and 74. Referring specifically to the flexure 68, the device includes outer flexible beams 76 and 78 that are anchored to the stator (not shown) and an inner flexible beam 80 that is connected to the translator. All three of the flexible beams have ends that connect to a floating rigid beam 82 of the type described above. The technique for at least partially offsetting the attractive force that is generated by interaction of translator drive electrodes 84 with the stator drive electrodes (not shown) is to include levitator electrodes 86 on both the translator and the stator. Only eight translator levitator electrodes are shown in FIG. 6, but typically a larger number of such electrodes are included. The electrical connections to the levitator electrodes are shown in FIG. 7. An alternating pattern of voltage high and voltage low states is established along the levitator electrodes of the translator 66. A corresponding alternating pattern is established along the levitator electrodes 88 of the stator 90. Consequently, the translator is repulsed from the stator, providing levitation force.
For the purpose of clarity, the operation of the drive electrodes will be briefly described with reference to FIG. 8. The voltage pattern along the drive electrodes 84 of the translator 66 is fixed. While not critical, the voltage pattern is preferably an alternating pattern of electrical high and electrical low. On the other hand, the voltage pattern along the drive electrodes 92 of the stator 90 is varied to induce the in-plane movement along the x axis. The applied voltages generally alternate, but include a xe2x80x9cdisruptionxe2x80x9d in the alternating pattern. In FIG. 8, the disruption occurs at electrodes 94 and 96, since these adjacent electrodes are both tied to electrical high. By moving the disruption along the x axis, the translator 66 will be moved to a new equilibrium position, thereby providing the desired translator displacement.
As is clear from FIG. 8, the voltage patterns along the drive electrodes 84, 92, 94 and 96 will generate attractive forces. The levitator electrodes 86 and 88 of FIG. 7 are aligned parallel to the direction of travel, so that the relative electrode positions remain fixed as the translator 66 is displaced in a direction perpendicular to the drive electrodes. By placing the appropriate voltages on the levitator electrodes, it is possible to mitigate, and in some cases completely counteract, the attractive force produced by the drive electrodes. Thus, the levitator electrodes significantly ease the limits imposed on translator travel as a result of the previously described reductions in out-of-plane stiffness kz.
Unfortunately, the addition of the levitator electrodes 86 and 88 has the secondary effect of increasing the stiffness requirements with respect to in-plane displacements normal to the direction of travel, i.e., displacements along the y axis. The increase with regard to in-plane stiffness is apparent from FIG. 7. The desired repulsive forces are achieved by aligning the high voltage electrodes 86 of the translator 66 with the high voltage electrodes 88 of the stator 90. Without sufficient in-plane stiffness along the y axis, the desired alignment will be lost.
What is needed is an actuator that satisfies in-plane stiffness and out-of-plane stiffness requirements and provides a desired electrical relationship to enable a long range of motion without jeopardizing stability.
At least one flexure that is continuously flexible from a first end anchored to a stationary member to a second end fixed to a movable member may be combined with a levitation scheme to provide an actuator with a long range of motion. In the preferred embodiment, the flexures that are continuously flexible between their ends are of equal length and, at least in the most preferred embodiment, are straight beam flexures. Straight beam flexures are at least one order of magnitude stiffer than folded beam flexures. This greater stiffness enables longer ranges of travel.
In one preferred embodiment, the movable member is a translator of an electrostatic surface actuator which provides generally straight-line motion. The translator is supported by four straight beam flexures. The flexures are of equal length and extend in the same direction from a rigid connection to the translator. As the translator moves laterally along the x axis, each flexure bends, causing the translator to be displaced an amount xcex4y in the y direction. This displacement is related to the lateral movement xcex4x and the beam length L as follows:
xcex4y≅0.6xcex4x2/Lxe2x80x83xe2x80x83Eq. 2
Because the beams are the same length, the orientation of the translator does not change as it is displaced. Moreover, even though the beams are bent, the beams remain extremely stiff with respect to forces in the y direction.
Both the translator and the stator include arrays of drive electrodes and levitator electrodes. The drive electrodes are physically configured and electrically manipulated in the same manner described with reference to FIG. 9. However, the levitator electrodes are configured in an unconventional manner. Preferably, the pitch of the levitator electrodes on the translator is different than the pitch of the levitator electrodes on the stator, with the pitch being defined as the average center-to-center distance between the electrodes. There may be 2nxc2x11 electrodes in a repeating group of stator levitator electrodes for every 2n translator levitator electrodes. In this configuration, a strictly alternating voltage pattern is applied to the translator levitator electrodes, while a disruption in the alternating pattern is applied along the stator levitator electrodes. Unlike the drive electrodes, the voltage pattern along the stator levitator electrodes is selected so that most translator levitator electrodes are positioned in alignment with stator levitator electrodes biased at the same voltage. The levitator electrodes then produce a force pushing the translator away from the stator. In the most preferred embodiment, the translator pitch is smaller than eight times the distance g between the translator and stator. This pitch-to-distance arrangement provides both a large drive force and a large levitating force.
In this first embodiment, the drive electrodes are used to step the translator in the desired x direction of travel. As the translator is displaced in the x direction, the bending of the straight beams causes the actuator to also move in the y direction. The y displacement xcex4y depends quadratically on the x displacement xcex4x. Typically, the y displacement is parallel to the drive electrodes and is much smaller than the drive electrode length, so that the drive method is not affected by the displacement. However, the y displacement does cause the levitator electrodes to move relative to each other. To accommodate this movement, the voltage pattern on the stator levitator electrodes is preferably stepped, keeping the desired alignment of opposing voltages for the two arrays of levitator electrodes.
In a second embodiment, the translator is supported by four continuously flexible beams that are curved when in a relaxed condition. That is, the beams are pre-bent. This reduces the available travel in one direction along the x axis, but doubles the possible throw in the opposite direction, which may be desirable in some applications. The physical and operational arrangements of the drive electrodes and the levitator electrodes of the first embodiment apply equally to this second embodiment. One method of producing pre-curved beams is to use curved masks in the etching of the beams.
In a third embodiment, the translator is mounted for rotation about an axis that is intersected by the only straight beam flexure that is used to support the rotary translator. While not critical, the actuator is designed to rotate about an axis that is located at approximately 80% of the length of the straight beam flexure (i.e., 20% of the length as measured from the anchoring of the flexure to the stator). Folded beam flexures or other non-straight flexures are used to support the outer edge of the translator. In this embodiment, the drive electrodes extend along radial lines from the rotational axis, but are operated in the same manner as the straight-line embodiments described above. The levitator electrodes are curved and have a common center at the rotational axis. Also in this embodiment, the levitator electrodes do not require voltage pattern stepping, since the relative positions of the levitator electrodes of the two arrays remain fixed as the translator is moved.